Significance Analysis of mutants in the transmembrane of env (env-TM) domain of the pathogenic SIVmac239 clone may provide insight on the role of this domain in viral load and simian AIDS (SAIDS) pathogenesis in infected macaques. Information obtained from such in vivo studies will facilitate the development of live-attenuated lentivirus vaccines, and may also identify a novel target(s) for antiviral therapy. Objectives A large portion of the env-TM domain of SIV is dispensable for viral replication in vitro in tissue culture cells. The main objective is to determine the importance of this domain in vivo by inoculating rhesus macaques with viruses containing point mutations in intracellular portion of the env-TM of SIVmac239. Results Mutations (missense and stop codons) in the env-TM of SIVmac239 reduced replication of the respective virus in cultures of peripheral blood lymphocytes from macaques. Four macaques inoculated with a viral clone containing a premature stop codon in env-TM exhibited high virus load and are showing signs of immunodeficiency disease. DNA sequence analysis revealed rapid reversion of the stop codon. Macaques inoculated with a viral clone with multiple mutations in the env-TM domain are displaying low virus load with no signs of disease. Future Directions Biochemical experiments will be performed to identify a cellular protein(s) that interacts with the intracellular portion of env-TM. Importantly, SHIV clones with mutations in the HIV-1 env-TM will be constructed and tested for virus load and pathogenic potential in rhesus macaques. KEY WORDS simian AIDS, SIV clones, env mutants FUNDING NIH Grant AI39415